How to Capture HTTP Servlet Response Output Stream for Analysis?

Logging HTTP Servlet Response Output Stream for Analysis

To effectively log the HTTP servlet response output stream content for further analysis, a comprehensive strategy is required. This approach involves leveraging a custom filter that wraps the ServletResponse argument with a tailored HttpServletResponseWrapper implementation. This wrapper overrides the getOutputStream() and getWriter() methods, returning a customized ServletOutputStream implementation.

Implementation Details

The goal of the filter is to capture the ServletOutputStream in a separate ByteArrayOutputStream, effectively creating a copy of the response output stream. This allows for capturing the actual string or content response.

Specifically, the FilterResponseCopier class is created to wrap the HttpServletResponse object. Inside the doFilter method, the request and the FilterResponseCopier response are passed to the FilterChain#doFilter() call. Subsequently, the getCopy() method of the FilterResponseCopier can be utilized to retrieve the copied response after the filter chain has completed execution.

Custom HttpServletResponseWrapper

The HttpServletResponseCopier class serves as the backbone for customizing the HttpServletResponse object. It maintains references to the ServletOutputStream, PrintWriter, and ServletOutputStreamCopier.

Overridden getOutputStream() and getWriter() Methods

These methods ensure that the ServletOutputStreamCopier instance is always returned, providing access to both the original output stream and the copy of that output stream.

Custom ServletOutputStream

The ServletOutputStreamCopier class is responsible for intercepting the written byte(s) in its base abstract OutputStream#write(int b) method. It simultaneously writes these bytes to both the original output stream and the copy.

Logging the Copied Response

With the intercepted byte(s), you can access the copied response and perform the desired logging through any means necessary, be it console output or database insertions. This allows you to capture and analyze the servlet's response output stream effortlessly.

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